Re: [CAD_CAM_EDM_DRO] Re: Knee for z-axis control? pnuematic actuator

Stephen Wille Padnos wrote:

>Are those calculations for a knee that's driven by air, or for one that
>just has an air "spring" to reduce the load on a servo motor? In a
>cylinder/accumulator system, do you lose the air from the piston when
>it's moved? I would think that a sealed shock absorber-like system
>would be best for the servo/spring combination. (I thought that was
>what the question was)
>
>

The system Bridgeport uses in the Series-II CNC machines is a cylinder
and a relieving-type regulator. Every time the knee is lowered, the
pressure
in the cylinder rises, and the regulator lets a pffft of air out to maintain
pressure. When the knee goes up, air is needed from the compressor.

If the cylinder were hooked to a closed accumulator system, the accumulator
would need to have something like 10 x the volume swept by the cylinder to
keep the pressure change small. For instance, with a 4" cylinder
experiencing
a travel of 17", that would sweep .124 Cu-Ft. If the accumulator had a
volume of 1.24 Cu-Ft, the pressure would vary from, say, 5 Bar absolute
at the
top to 5.5 Bar at the bottom. Assuming you had a 754 Lb knee that was
perfectly balanced at the top of stroke, it would have an additional 7% Lb
of support at the bottom. So, it seems you don't need a real big
accumulator
at all. With a totally closed system, if the cylinder slowly lost air
as the
seal wore, you wouldn't know anyting was wrong until the servo faulted
out on an upward move. (Well, I guess you'd put a pressure gauge in
it, and you could even hook a pressure switch to the E-stop chain.)

Jon